Exercise devices are used for a variety of purposes: to increase the strength and improve the tone of the targeted muscle groups; to increase the bulk of same; to increase the endurance of same; and to increase the endurance and promote the fitness of the body generally, especially the cardiovascular system (heart and lungs). Many devices try to fulfill a combination of these purposes.
In order to achieve their objective, exercise devices perform one or both of two functions: they can offer resistance to bodily movements and they can regulate and motivate such movements. The resistance function is best exemplified by barbells and springs. The regulation-motivation function is exemplified by jump ropes and threadmills.
The resistance function of weights can be intensified by using momentum. This can be done by imparting motion to a mass and changing the velocity or direction of this motion by muscular action. This principle is exemplified by a few exercisers patterned after the "button-on-a-string" toy in which a disk is made to rotate alternately in clockwise and counter-clockwise directions.